As used herein, "liquid" is intended to include all materials susceptible of being spread, smoothed out and conveyed by rolls, such as semi-liquids, pastes, oily masses and the like.
Printing and various other processes involving the deposition of a thin film of liquid in a controlled manner from a supply reservoir onto a receiving surface often employ a liquid distribution system which utilizes a train of distribution rolls for workng and rolling out the liquid in a controlled manner to provide a uniform film having desired characteristics. In publication printing, for example, the thickness of the print on one side of a printed page is on the order of 0.0002 in., or so. The extreme thinness of the ink film to be produced requires considerable care in inker design and operation.
In a typical such process, a slowly rotating fountain roll, partially immersed in a liquid reservoir, is used to initially pick up an amount of the liquid from the reservoir. The liquid is then transferred from the fountain roll onto a succession of high speed rotating distribution rolls which serve to spread it out before applying it to the receiving surface. Because of the difference in speeds between the slow fountain roll and the fast distribution rolls, such systems usually require some sort of transfer means other than direct contact between the fountain roll and the first distribution roll.
One form of transfer means frequently used is a freely rotating ductor roll which is positioned in a gap between laterally-spaced parallel fountain and first distribution rolls, and which shifts back and forth between a position of roller contact with the fountain roll and a position of roller contact with the first distribution roll. In a typical arrangement, the ductor turns first several times in contact with the fountain roll picking up a layer of liquid therefrom, then shifts into contact with the first distribution roll for several rotations, depositing the liquid in a mat thereon before shifting back to the fountain roll. Because of its intermittent action and smaller outside diameter, the ductor brings up mats of liquid in successive waves longitudinally across the first distribution roll. Such mats are then spread out and overlapped into a uniform film as they are passed from distribution roll to distribution roll. The distribution procedure is enhanced by providing counterrotating rolls of different diameters and by vibrating some of the rolls longitudinally relative to the others. The train of distribution rolls is organized so that the last distribution roll or rolls (called the "form" rolls in printing) will transfer a film of the liquid of desired thickness and uniformity onto an ultimate receiving surface, such as the surface of a printing plate cylinder or drum. Control over pick-up of liquid onto the fountain roll, and therefore over the deposited film, is afforded among other things by varying the speed of rotation of the fountain roll.
The use of such ductor rolls has many disadvantages. The intermittent transfer of liquid in a ductor system complicates the liquid distribution process. A ductor may spend one-third of its time picking up ink from the fountain roll, one-third of its time delivering ink to the distribution roll, and one-third of its time travelling from one roll to the other. Thus, for every cycle of the ductor, there is a lapse in the ink feed which must be compensated for by depositing a heavier film on the distribution roll which must be levelled out to cover the gaps. Moreover, liquid is transferred by the ductor in wedge-shaped patches of nonuniform thickness. More liquid is transferred when the ductor traveling at the slower fountain roll speed first contacts the distribution roll, than after it adjusts to the higher distribution roll speed.
Because of the way the ductor transfers liquid from the fountain to the distribution train in mats, a large number of rollers is needed to smooth and spread the liquid into the desired film uniformity and thickness. The liquid must be kept at the right consistency and be prevented from evaporating excessively over such long distribution paths. This is particularly important in printing, where the ink has to be kept from drying before it reaches the plate. Evaporation of ink solvents over such long inking paths is also a problem from an environmental standpoint, because of the adverse effect of the vapors on the Earth's ozone layer that provides protection against the harmful effects of the sun's radiation. In high speed presses, shifting a large heavy ductor back and forth between a fountain roll turning at, say, 800 ft. per min. and a first distribution roll rotating at, say, 2,000 ft. per min., expends much energy and generates considerable heat. Heat not only increases evaporation but also affects the viscosity of the distributed liquid. The use of expensive specially formulated, high boiling point narrow range solvents is thus necessitated.
The drying process in high speed printing is also complicated by the use of a ductor distribution system. As the mats of ink are transferred from roll to roll, there is nonuniform exposure of the ink solvents to air and spotty areas of higher retained solvent content remain after printing. This nonuniformity, as well as the general requirement for higher solvent boiling points, necessitates the use of higher velocity air blasts and more heat in the dryer. Increased air velocity or other measures must consequently be taken during drying to prevent browning of the printing medium, especially when the press stops. Also, since the printing medium is heated to a greater extent, the cooling step will require greater cooling.
Conventional liquid distribution systems of the type to which the present invention relates, especially those involving high viscosity liquids such as printing inks, will typically further control the transfer of liquid from the reservoir to the fountain roll by means, such as a flexible blade, extending longitudinally across the fountain and means for selectively adjusting the spacing of the blade from the fountain roll to provide predetermined varying degrees of liquid pick-up longitudinally across the roll. In a typical present day high speed printing press inker arrangement, for example, a blade is positioned adjacent where the fountain roll emerges from the fountain, with the blade forming an extension of the bottom of the fountain. Normal spacing of the blade from the fountain may be about 0.015 in., with keys being provided at spaced intervals of about 11/4 in. longitudinally of the blade to enable local adjustment of the spacing. Closer spacing of the blade reduces the ink coating picked up and carried by the fountain roll at a particular point; wider spacing increases the thickness.
The keys are individually adjusted as needed to provide the required thickness and uniformity of the deposited liquid film at the receiving surface. On high speed presses, some keys are adjusted to bring the blade into close abutment with the fountain roller, to thereby prevent the inking of certain locations of the receiving surface, such as those corresponding to the margins of the pages to be printed.
The action of the blade in controlling the liquid transfer to the fountain roller serves as another source of heat generation in the liquid distribution system. This is especially true where the keys are adjusted locally to inhibit any ink pick-up. Local variations in heat generation lead to undesirable local variations in liquid viscosity for which compensation at the receiving surface is difficult. Overall and local heat generation also causes thermal expansion of the blade which changes the distance of the blade from the fountain roller, requiring frequent key readjustment. Changes in gap of 0.001 in. due to heating are not unusual. Local variations in heating and keying can also have temporary and permanent distorting effects on the blade which interfere with the liquid film thickness control.
The blade is usually of tempered steel. Pressure of the blade against the fountain roll can also cause the roll itself to deflect. Fountain roll deflection will affect the thickness settings across the roll in unpredictable ways, changing the effect of other key settings. Reducing the gap between the blade and the roll at one location may, for example, because of deflection of the roll, cause completely unwanted liquid flow changes at adjacent key locations.
Prior art liquid distribution systems also have the disadvantage of providing a return path from the receiving service to the reservoir for the passage of dust, dirt, old liquids and similar undesirable substances which contaminate the fresh liquid supply. In printing systems, for example, paper lint, stale solvents and other dirt from the plate may work their way back to the ink fountain where they contaminate and change the character of the ink. This is apparent when the ink loses its translucent quality and becomes darker. Although printing may still be possible, the thickness of the film on the plate may have to be increased significantly because the impurities have no color value. This is accomplished by speeding up the fountain roll, moving the fountain blade outward, or both. The accumulation in the fountain of paper debris that works its way back through the ductor can be a formidable problem, requiring constant attention. Debris accumulation in the fountain of a high speed press may approach one-third the volume of the ink reservoir after just a few hours of press run time.
Various proposals have been made to address the problems of conventional ductor liquid distribution systems. To reduce the inertial effect and wear in the shifting of the ductor roller back and forth between the fountain roll and the first distribution roll, the ductor roll has been configured with a plurality of eccentric ring sections, so that some of the ring sections are in contact with the fountain roll while others are in contact with the distribution roll. Examples of such eccentric section ductor arrangements are shown in U.S. Pat. Nos. 3,037,449 and 4,467,720. In such arrangements, although the overall ductor roll remains in contact at all times with both the fountain roll and the distribution roll, individual roller sections of the ductor shift their contact between the two rolls.
My U.S. Pat. No. 3,261,287 follows a different approach and proposes a ductorless distribution system that transfers liquid between a slow speed fountain roll and a high speed distribution roll by means of spherical steel balls supported side-by-side by downwardly directed guides for free rotation in the space between the rolls. The balls all remain in contact with both rolls at all times. The balls act to bring up narrow lines of liquid from the fountain roll to the distribution roll. Relative longitudinal vibration between the ball guides and the distribution roll causes the liquid lines to be deposited on the distribution roll in the form of crisscrossing sinusoidal lines.
My U.S. Pat. No. 3,261,287 ductorless arrangement, which lays the ink down in lines rather than in mats as in conventional systems, permits the deposited liquid to be spread out and worked into a substantially uniform film of desired thickness with a much reduced number of rolls. There is also significant reduction in the debris returning from the receiving surface back to the reservoir. Ink thickness control of the '287 arrangement uses a key and blade setup, as discussed above. However, for locations where the complete absence of liquid is desired, balls may be removed and spaces left, thereby eliminating the need to key the blade into closed gap configuration at such locations.
The present invention builds upon the experience gained in developing my U.S. Pat. No. 3,261,287 arrangement, and provides an improved ductorless liquid distribution system that offers all the advantages of my U.S. Pat. No. 3,261,287 system over conventional ductor systems and offers additional advantages as well.